Device for measuring pressure in two points of a fluid flow

ABSTRACT

The invention concerns a device for measuring pressure in two points of a fluid flow, comprising: a frame ( 10 ) consisting of two plates ( 12, 14 ) comprising each two planar surfaces, one outer ( 12   a   , 14   a ) and the other inner ( 12   b   , 14   b ), and wherein one of the plates ( 12 ) is perforated with recess ( 22 ) closed by the other plate ( 14 ) to form an assembly of two chambers ( 28, 30 ) comprising two planar walls ( 24, 14   b ) parallel to the surfaces ( 12   a   , 14   a ) of the frame and a side wall ( 26 ) forming its periphery and a fluidic restriction channel ( 32 ) connecting the two chambers ( 28, 30 ) with each other, and means ( 16 ) for supplying a measurement of the pressure in each of the chambers ( 28, 30 ). In order to improve the accuracy of the measurement, the side wall ( 26 ) of the chambers ( 28, 30 ) is perpendicular to its two planar walls ( 24, 14   b ) and is configured such that the chambers ( 28, 20 ) are spindle-shaped.

This application is a 371 of PCT/CH02/00101, filed Feb. 21, 2002.

The present invention relates to devices for measuring pressure at twopoints of a fluid flow. It concerns, more particularly, a device of thetype including:

-   -   a frame formed of two plates each including two planar surfaces,        one outer and the other inner, the two inner surfaces being        mounted on each other, and wherein one of the plates is provided        with a recess closed by the other plate to form together two        chambers including two planar walls parallel to the surfaces of        the frame and a side wall forming its periphery and a fluidic        restrictor channel connecting the two chambers, said frame        including two apertures respectively connecting the two chambers        to the exterior to allow a fluid to flow therein, and    -   means for supplying a measurement of the pressure in each of the        chambers.

Such a device is, for example, described in the publications entitled“High Precision Piezo-Resistive Sensing Techniques for Micro-DosingApplications” (M. Boillat et al. Proceedings Sensor Expo Cleveland 1999)and “A Differential Pressure Liquid Flow Sensor for Flow Regulation andDosing Systems” (M. Boillat et al. 0-7803-2503-6© 1995 IEEE). The plateprovided with a recess is made of mono-crystalline silicon (100), therecess being made by chemical etching, whereas the other plate is madeof glass. The pressure measurement is carried out by means of twopiezo-resistive transducers respectively arranged on the outer surfaceof the etched plate facing the two chambers and connected in aWheatstone bridge configuration.

Because the recess is made by silicon etching, it has an isoscelestrapeze shaped section whose large base and sides form an angle ofapproximately 54°. Moreover, the side wall, defining a rectangularcontour, is formed of planar surfaces mounted on each other.

With such a structure, it can happen that, during filling by means of aliquid, gas bubbles remain trapped in one or other of the chambers andconsiderably affect the precision of the measurement because of theirelasticity.

It is thus an object of the present invention to provide a device thatprevents such bubbles from remaining trapped, and thus allows a pressuremeasurement offering maximum measurement precision and security to becarried out.

More precisely, the invention concerns a device of the type previouslydescribed, but which, in order to achieve the aforementioned object, ischaracterized in that the side wall of the two chambers is shaped suchthat they are spindle-shaped, or in other words fusiform.

Advantageously, the side wall of the two chambers is substantiallyperpendicular to its two planar walls. Moreover, the two chambers arealigned along the longitudinal axis of the frame and arrangedsymmetrically with respect to its transverse axis.

According to a preferred embodiment, the bottom wall of the recess formsa membrane that can be deformed elastically via the effect of thepressure in each of the two chambers and the measurement means includetwo electromechanical transducers respectively arranged facing thechambers, outside the membrane. In this case, each transducer isadvantageously formed of two pairs of piezo-resistors, interconnected soas to form a Wheatstone bridge and arranged symmetrically with respectto the longitudinal axis of the frame, the two piezo-resistors of eachpair being arranged symmetrically with respect to the transverse axis ofthe chamber.

Finally, it is advantageous to provide the device with a temperaturesensor arranged on one of the plates in proximity to the fluid flow suchthat the measured temperature corresponds to that of the liquidconcerned. It is thus possible to determine the liquid flow rate withprecision.

Other features and advantages of the invention will appear from thefollowing description, made with reference to the annexed drawing, inwhich:

FIGS. 1 and 2 show, respectively plan and cross-sectional views of adevice according to the invention, and

FIGS. 3 and 4 show the base plate of the device, seen respectively fromabove and below.

The drawing shows a device including an envelope 10 formed of a baseplate 12 made of mono-crystalline silicon and a glass strip 14 acting asa cover. Plate 12 and strip 14 each include two large planar surfaces,one outer, identified by the letter a, and the other inner, identifiedby the letter b and mounted on each other by welding.

Envelope 10 is of generally parallelepiped shape, 10 mm in length alonga longitudinal axis A—A, 3 mm wide along a transverse axis B—B, andapproximately 1 mm thick, with large surfaces respectively defined bythe outer surfaces 12 a of plate 12 and 14 a of strip 14, parallel toeach other and to the assembled surfaces 12 b and 14 b. The othersurfaces of envelope 10 are formed by the edges of plate 12 and strip14.

Envelope 10 carries, on its outer surface 12 a (FIG. 3), eightpiezo-resistors 16 arranged symmetrically in two groups of four withrespect to the transverse axis B—B of the parallelepiped. Each group isformed of two pairs of piezo-resistors arranged symmetrically withrespect to longitudinal axis A—A.

Envelope 10 also carries, on surface 12 a, a thermo-resistor 18 arrangedsubstantially in the middle of one of the large sides of theparallelepiped. This thermo-resistor is used to measure the temperatureof the device.

A network of conductive paths 20 connects the four piezo-resistors 16 ofeach group to each other so as to form two Wheatstone bridges formingtwo transducers whose function will appear hereinafter. These paths alsoensure the electrical connection of the two bridges, and that ofthermo-resistor 18, to terminals 21 arranged at both ends of surface 12a.

As FIG. 4 shows, surface 12 b of plate 12, intended to be mounted onplate 14, has a recess 22 whose bottom 24 is parallel to surfaces 12 aand 14 a, and whose wall 26, substantially perpendicular to bottom 24,is shaped so as to form, with surface 14 b of plate 14, twospindle-shaped chambers, an inlet chamber 28, the other an outletchamber 30, aligned along longitudinal axis A—A of the device andarranged symmetrically with respect to its transverse axis B—B so thatthe median part of each is facing the four piezo-resistors 16 of eachgroup. It will be noted that the two piezo-resistors of each pair arearranged symmetrically with respect to the transverse axis of thechamber, parallel to axis B—B.

The two chambers 28 and 30 communicate with each other via a channel 32arranged in plate 12 to form a U-shaped fluidic restrictor.Thermo-resistor 18 is in immediate proximity to channel 32, such thatthe temperature of the plate at this place perfectly corresponds to thatof the liquid contained in the channel.

In its part comprised between wall 12 a and bottom 24, plate 12 has asufficiently small thickness, typically 15 μm, to form an elasticallydeformable membrane 33. It is on this membrane that piezo-resistors 16are arranged, such that a difference in pressure between the twochambers 28 and 30 can be measured by means of the electromechanicaltransducers formed by the two Wheatstone bridges.

Two cylindrical holes 34, made in strip 14 and oriented perpendicularlyto plate 12, form apertures for respectively connecting chambers 28 and30 to the exterior of envelope 10. These holes open out into thechambers at their ends opposite to fluidic restrictor 32.

In the device described, the liquid enters one of holes 34 and leaves bythe other, after passing through chamber 28, fluidic restrictor 32 andchamber 30.

Such a device is manufactured from mono-crystalline silicon wafers, likethose used for manufacturing integrated circuits. Several tens ofdevices can be manufactured simultaneously on each of these wafers.

The first step of the method consists in forming, on one of the surfacesof the wafer, piezo-resistors 16, thermo-resistor 18, conductive paths20 and terminals 21, by means of techniques used, conventionally, in themanufacture of integrated circuits, for example as described in the workentitled “Silicon sensors” by S. Middelhoek et al. (ISBN 0-12-495051-5).

The wafer is then turned over to etch chambers 28 and 30 and fluidicrestrictor 32. This operation can, advantageously, be achieved by DRIE(Deep Reactive Ion Etching). This technique, described by P.-A. Clerc etal, in J. Micromech. Microeng. 8(1998) 272–278, allows recesses with adepth of up to 500 μm to be made. It also allows a fluidic restrictor 32having a width of 50 μm to be made. It is, consequently, possible toobtain a low ratio between the section of restrictor 32 and that ofchambers 28 and 30, which improves the measurement precision. Thistechnique also allows walls forming, with the surfaces, an angle greaterthan 85°, i.e. substantially perpendicular, to be obtained.

Plate 12 and strip 14 are then prepared for assembly by anodic welding.The way in which such an assembly is achieved is described in the workentitled “Process development for 3D silicon microstructures” by EricPeeters (Catholic University of Louvain). When this operation iscompleted, the wafer is sawn so as to separate the devices from eachother.

The device as described allows not only a difference in pressure and atemperature to be measured, but also a flow rate. Indeed, as specifiedin the aforementioned publication by M. Boillat et al., the flow rateinto a calibrated conduit is a function of the geometric dimensions ofthe conduit, the pressure difference and the viscosity of the liquid.Thus, for a given liquid, from the information collected via thethermo-resistor, it is possible to determine the liquid viscosity, thelatter varying only as function of temperature. Transmission ofinformation relating to the temperature and the pressure differencebetween the two chambers to an electronic measuring circuit thus allowsthe flow rate of the liquid concerned to be defined.

The device described, with a restrictor 32 having a length of 10 mm, awidth of 50 μm and a height of 150 μm, allows flow rates of up to 10μl/s to be measured, while remaining in laminar mode. It goes withoutsaying that these features can vary significantly. They are selected asa function of the liquid concerned and its flow rate.

Thus, precision can be improved by increasing the length of therestrictor or by reducing the section. This results in an increase inthe flow resistance, which requires higher pressure, to obtain the sameflow. The maximum admissible flow rate is thus reduced.

1. Device for measuring pressure at two points of a fluid flow,including: a frame (10) formed of two plates (12, 14) each including twoplanar surfaces, one outer (12 a, 14 a) and the other inner (12 b, 14b), the two inner surfaces being mounted on each other and wherein oneof the plates (12) is provided with a recess (22) closed by the otherplate (14) to form together two chambers (28, 30) including two planarwalls (24, 14 b) parallel to the surfaces (12 a, 14 a) of the frame anda side wall (26) forming its periphery and a fluidic restrictor channel(32) connecting the chambers (28, 30) to each other, said frame (10)including two apertures (34) respectively connecting the two chambers(28, 30) to the exterior to allow a fluid to flow therein, and means(16) for supplying a measurement of the pressure in each of saidchambers (28, 30), characterized in that the side wall (26) of the twochambers (28, 30) is shaped such that they are spindle-shaped.
 2. Deviceaccording to claim 1, characterized in that the side wall (26) of thetwo chambers (28, 30) is substantially perpendicular to its two planarwalls (24, 14 b).
 3. Device according to claim 1, characterized in thatthe two chambers (28, 30) are aligned along the longitudinal axis (A—A)of the frame (10) and arranged symmetrically with respect to itstransverse axis (B—B).
 4. Device according to claim 1, characterized inthat the bottom wall of the recess (22) forms an elastically deformablemembrane (33) via the effect of the pressure in each of the two chambers(28, 30) and in that said means include two electromechanicaltransducers respectively arranged facing said chambers, outside saidmembrane.
 5. Device according to claim 4, characterized in that eachtransducer is formed of two pairs of piezo-resistors (16) interconnectedso as to form a Wheatstone bridge and symmetrically arranged withrespect to the longitudinal axis (A—A) of the frame, the twopiezo-resistors of each pair being symmetrically arranged with respectto the transverse axis of the chamber.
 6. Device according to claim 1,characterized in that it further includes a temperature sensor (18)arranged on one of said plates, facing said channel (32).